A common type of keyboard-type input device includes an array of signal lines arranged in a series of overlapping rows and columns. The rows and columns of signal lines are positioned to closely pass by each other at a plurality of intersecting positions, with each of the intersecting positions being situated beneath a key on the keyboard. Pressing a key on the keyboard causes the row line and column line passing each other at the intersecting position beneath the key to come in contact with one another, thus closing a switch formed by the row line and the column line passing beneath that key. In compact devices, the keyboard typically includes two sets of foil strips aligned at right angles to each other, such that when a key is pressed, the intersecting foil strips are pressed together to close the switch at the point of intersection.
In a conventional process for scanning such a keyboard, a signal is applied sequentially to each of one of the sets of signal lines while reading the opposing set of signal lines to determine which key or keys have been pressed. For example, a signal in the form of a high-level voltage representing a binary 1 may be sequentially applied to each of the column lines. The row lines are monitored to detect a signal indicating that one or more of the row lines presents the high-level signal, indicating that row line has become electrically coupled with a column line to which the high-level signal has been applied. By detecting the change on one of the row lines and determining to what column the signal was applied, one can determine which column line and which row line came in contact to identify which key was pressed.
Conventional keyboard scans, however, have shortcomings. First, and of particular concern in battery-powered devices, the longer that a signal has to be applied in identifying which key is pressed, the more power will be consumed in scanning the keyboard. Particularly when multiple keys are pressed simultaneously, both the current flowing through the coupled signal lines and the current used to power the keyboard scanning system consume appreciable power. Second, the signals applied to the keyboard may result in uncontrolled short-circuits. In compact devices using foil-type keyboard, the closely-spaced foil strips may short-circuit, thereby both wasting power and give false key press signals. Third, the pressing of a key can result in signal crosstalk between adjacent lines that leads to erroneous identification of which key or keys were pressed. Signal crosstalk is common in compact, foil-type keyboards because of the short distances between the row lines and column lines in contrast to the relatively much longer signal lines they may connect.
In the drawings, the first digit in three-digit reference numerals and the first two-digits in four-digit reference numerals refer to the figure in which the referenced element first appears.